1. Field of the Invention
The present invention relates to electrophotographic image-forming apparatuses, for example, copying machines and laser beam printers.
2. Related Background Art
In a typical electrophotographic image-forming apparatus, image formation can be achieved by the following process. A toner image is formed on a photosensitive drum serving as an image carrier by toner-image forming means constructed of a charger, an exposure unit, and a developer unit. Then, the formed toner image is transferred by transfer means to a transfer material, such as a sheet of paper, which is then separated from the surface of the photosensitive drum by separating means. Subsequently, the toner image on the transfer material is fixed by a fixing unit.
As the above-described separating means, a separating charger and a discharging needle are known by way of example. Further, a combination of the above-mentioned separating charger or the discharging needle and a separating claw used as a detaching member is known as the separating means. This claw is located downstream of the separating charger or the discharging needle and upstream of a cleaner in a direction in which the drum is rotated. The claw separates a transfer material from the surface of the photosensitive drum by inserting its tip end between the surface of the drum and the forward end of the transfer material. It is required that the separating claw be constantly in touch with the surface of the photosensitive drum during a copying operation.
Meanwhile, during a start-up operation of an image-forming apparatus (when a main power supply is switched on) or during an idling operation, i.e., an occasion other than a copying operation, a developing sleeve of the developer unit is rotated and a bias voltage is applied to the developing sleeve to allow toner with unstable electric charges (hereinafter referred to as "unstably charged toner"), such as inverting toner or fog toner, within a developer unit to adhere to the photosensitive drum, thereby substantially maintaining a constant developing density (preventing a change in density). The unstably charged toner includes toner having low tribo-electricity charged with a polarity opposite to a charging polarity of the normal toner, and toner having tribo-electricity lower than the normal toner.
Accordingly, during an occasion, such as a start-up operation and an idling operation, other than copying operation of the image-forming apparatus, the inverting toner or the fog toner attached to the surface of the photosensitive drum transferred from the developer unit is unfavorably caused to adhere to the separating claw serving as a contact member. This toner may disadvantageously drop onto the charger or images formed on the initial few transfer sheets and deface them when a copying operation is performed, i.e., what is referred to as "toner dropping" is generated.
To solve this problem, the following measures may be taken: PA1 (1) providing a contact-releasing mechanism for the separating claw used as the contact member being in contact with the surface of the photosensitive drum; the mechanism is actuated to separate the claw from the surface of the photosensitive drum when the developing sleeve is rotated to apply a developing bias thereto during a start-up operation or an idling operation, and once again to bring the claw into contact with the surface of the drum when a copying operation is started; and PA1 (2) performing a specific sequence in which a process of rotating and biasing a developing sleeve is omitted depending on the degree of reduction in developer density in order to avoid "toner dropping" from the separating claw.
However, despite the aforedescribed measures, the following problems still remain. The provision of a contact-releasing mechanism for the separating claw proposed in (1) increases the number of parts and the complexity of the construction of the image-forming apparatus. The specific sequence described in (2) still exhibits a reduction in density, which does not solve the fundamental problem.